Poly (ADP-ribose) polymerase (PARP-1), a nuclear enzyme that facilitates DNA repair, may be instrumental in acute neuronal cell death in a variety of insults including cerebral ischemia, MPTP-induced parkinsonism, and CNS trauma. Excitotoxicity is thought to underlie these and other toxic models of neuronal death. Different glutamate agonists may initiate different downstream pathways in mechanisms of neurotoxicity. We examine the role of PARP-1 in NMDA and non-NMDA mediated excitotoxicity. NMDA and non-NMDA agonists were stereotactically delivered into the striatum of mice lacking PARP-1 and control mice in acute (48 hour) and chronic (3 week) toxicity paradigms. Mice lacking PARP-1 are highly resistant to the excitoxicity induced by NMDA, but are equally susceptible to AMPA excitotoxicity as wild type. Restoring PARP-1 protein in mice lacking PARP-1 by viral transfection restored susceptibility to NMDA supporting the requirement of PARP-1 in NMDA neurotoxicity. Furthermore, Western blot analyses demonstrate that PARP-1 is activated following NMDA delivery, but not following AMPA administration. Consistent with the theory that nitric oxide (NO) and peroxynitrite are prominent in NMDA-induced neurotoxicity, PARP-1 was not activated in mice lacking the gene for neuronal NO synthase (nNOS) following NMDA administration. These results suggest a selective role of PARP-1 in glutamate excitoxicity, and strategies of inhibiting PARP-1 in NMDA-mediated neurotoxicity may offer substantial acute and chronic neuroprotection. Poly (ADP-ribose) polymerase (PARP-1) is a nuclear enzyme that is activated primarily by DNA damage. Upon activation, the enzyme hydrolyzes NAD to nicotinamide and transfers adenosine diphosphate (ADP) ribose units to a variety of nuclear proteins including histones and PARP-1 itself. This process is important in facilitating DNA repair. However, excessive activation of PARP-1 can lead to significant decrements in NAD and ATP depletion which leads to cell death. Excessive PARP-1 activation is implicated in a variety of insults including cerebral ischemia, MPTP-induced parkinsonism, traumatic spinal cord injury and streptozotocin-induced diabetes. Excitotoxicity is thought to play a prominent role in a variety of acute and chronic neurologic injuries from the excessive activation of the excitatory neurotransmitter glutamate acting on N-methyl-D-aspartate (NMDA) receptor and non-NMDA receptors. A number of studies demonstrate a prominent role for nitric oxide (NO) in excitotoxicity in vivo and in vitro . Primary brain cultures treated with NO synthase (NOS) inhibitors or cultures from mice with targeted disruption of neuronal NOS (nNOS) are resistant to NMDA neurotoxicity . nNOS knockout mice are also resistant to neuronal damage following middle cerebral artery occlusion and intrastriatal NMDA excitotoxic lesions, but not AMPA excitotoxicity. NO is thought to mediate the majority of its toxic effects through the interaction with the superoxide anion to form the potently toxic oxidant peroxynitrite. Peroxynitrite and NO damage DNA, the presence of which is a prime activator of PARP-1. PARP-1 plays a key role in NMDA and NO-induced neurotoxicity as mice lacking the gene for PARP-1 or cultures treated with PARP-1 inhibitors are resistant to the toxic effects of these agents. The role of PARP-1 in excitotoxicity in vivo is not known; and more specifically, the contributions of PARP-1 to NMDA versus non-NMDA mediated neuronal damage have not been explored. In this study we demonstrate that NMDA excitotoxicity is dramatically reduced in PARP-1 knockout mice whereas PARP-1 knockout mice are sensitive to AMPA excitotoxicity.